The present invention relates to a conjugated diene-based rubber capable of being converted into a vulcanized rubber having a low rolling resistance, an excellent wet skid property sufficient wearing resistance, tensile strength and the like when incorporating an inorganic filler such as silica as a reinforcing agent, and also to an oil extended rubber containing such rubber and an extending oil. The present invention also relates to a rubber composition comprising a conjugated diene-based rubber or an oil extended rubber described above and an inorganic filler and having an excellent processability. This rubber composition gives a vulcanized rubber which is useful especially as a tire tread.
In response to recent demand for low gas mileage of automobile, rubber material such as a conjugated diene-based rubber or the like capable of giving a rubber composition for a tire which has low rolling resistance, excellent wearing resistance and destruction characteristics and whose wet skid resistance, which is a representative index of a driving stability, is high is desired.
Tire rolling resistance can be reduced by lowering hysteresis loss of vulcanized rubber. The hysteresis loss can be evaluated based on various properties. For example, compositions preferably have high impact resilience at 50-80xc2x0 C., a low tan xcex4 at 50-80xc2x0 C. or low Goodrich heat release. Compositions with low hysteresis loss include natural rubber, isoprene rubber, butadiene rubber and the like, but these are also associated with the problem of low wet skid resistance.
In recent years, there have been proposed methods of using inorganic filler such as silica and the like to serve as reinforcing agent, or combining inorganic filler with carbon black. Tire treads employing inorganic filler or both inorganic filler and carbon black have low rolling resistance and excellent driving stability, represented by, for example, the wet skid resistance. However, they exhibit a problem of poor wearing resistance, tensile strength and the like for the vulcanized rubber. The cause is believed to be the fact that the affinity of inorganic filler for the conjugated diene-based rubber is lower than that of carbon black, such that a sufficient reinforcing effect cannot be achieved.
In order to increase the affinity between the silica as an inorganic filler and the conjugated diene-based rubber, it has been attempted to use conjugated diene-based rubbers introduced therein functional group with affinity for inorganic filler. For example, there have been proposed a conjugated diene-based rubber introduced a hydroxyl group therein (WO96/23027), a conjugated diene-based rubber introduced an alkoxysilyl group therein (Japanese Patent laid-open publication No. HEI-9-208623) and a conjugated diene-based rubber introduced an alkoxysilyl group and an amino and/or a hydroxyl group therein (Japanese Patent laid-open publication No. HEI-9-208633). However, most conjugated diene-based rubbers with such an introduced functional group exhibit strong interaction with an inorganic filler when the inorganic filler is combined therewith, and this creates such problems as impaired dispersion of the inorganic filler, greater heat release during processing, poor processability and the like.
The present invention solves the problems associated with prior arts described above and is intended to provide a conjugated diene-based rubber and an oil extended rubber, each of which is capable of being converted into a vulcanized rubber having a low rolling resistance, an excellent wet skid property and sufficient wearing resistance, tensile strength, and the like. Another aspect of the invention is intended to provide a rubber composition capable of being converted into a vulcanized rubber which has an excellent processability and is useful as a tire tread of an automobile or the like.
A rubber composition for a tire tread and the like usually contains a reinforcing agent for the purpose of improving tensile strength and wearing resistance, but silica and similar substances do not easily disperse uniformly because of their tendency to aggregate. When using a rubber composition in which the reinforcing agent is not uniformly dispersed, the desired effect of including the reinforcing agent cannot be achieved, while the processability is also notably impaired. For this reason, it has generally been attempted to improve the dispersion properties by also including a silane coupling agent when using a silica as an inorganic filler.
It was discovered that by using a conjugated diene-based rubber having a certain composition in which an unsaturated nitrile monomer unit is contained in a molecular chain a sufficient improvement in the wearing resistance, the tensile strength, and the like as well as the processability can be achieved even with a reduced amount of a silane coupling agent or rather without any silane coupling agent.
The present invention is based on the findings described above and can be described as follows.
1. A conjugated diene-based rubber being characterized in having a repeating unit comprising {circle around (1)} 1 to 30% by mass of an olefinically unsaturated nitrile monomer unit, {circle around (2)} 10 to 50% by mass of an aromatic vinyl monomer unit {circle around (3)} 19.9 to 88.9% by mass of a conjugated diene monomer unit and {circle around (4)} 0.1 to 10% by mass of a monomer unit having one polymerizable unsaturated group and at least one functional group selected from the group consisting of an amino group, a hydroxy group, an epoxy group, a carboxyl group and an alkoxysilyl group [where the total of {circle around (1)}, {circle around (2)}, {circle around (3)} and {circle around (4)} is 100% by mass], and whose Mooney viscosity [ML1+4 (100xc2x0 C.)] is 20 to 200.
2. A conjugated diene-based rubber according to 1 above wherein, the glass transition point of the above-mentioned conjugated diene-based rubber is xe2x88x9260 to 0xc2x0 C., and the difference between the extrapolated temperatures of the initiation and the termination of the above-mentioned glass transition is 20xc2x0 C. or less.
3. A conjugated diene-base rubber according to 2 above wherein,
a monomer forming the above-mentioned olefinically unsaturated nitrile monomer unit is at least one selected from the group consisting of acrylonitrile and methacrylonitrile;
a monomer forming the above-mentioned aromatic vinyl monomer unit is at least one selected from the group consisting of styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, xcex1-methylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 4-tert-butylstyrene and tert-butoxystyrene;
a monomer forming the above-mentioned conjugated diene monomer unit is at least one selected from the group consisting of 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and chloroprene.
4. An oil extended rubber being characterized in comprising 100 parts by mass of a conjugated diene-based rubber which has a repeating unit comprising {circle around (1)} 1 to 30% by mass of an olefinically unsaturated nitrile monomer unit, {circle around (2)} 10 to 50% by mass of an aromatic vinyl monomer unit {circle around (3)} 19.9 to 88.9% by mass of a conjugated diene monomer unit and {circle around (4)} 0.1 to 10% by mass of a monomer unit having one polymerizable unsaturated group and at least one functional group selected from the group consisting of an amino group, a hydroxy group, an epoxy group, a carboxyl group and an alkoxysilyl group [where the total of {circle around (1)}, {circle around (2)}, {circle around (3)} and {circle around (4)} is 100% by mass], and whose Mooney viscosity [ML1+4 (100xc2x0 C.)] is 40 to 200, and 10 to 60 parts by mass of an extending oil.
5. An oil extended rubber according to 4 above wherein, the transition point of the above-mentioned conjugated diene-based rubber is xe2x88x9260 to 0xc2x0 C., and the difference between the extrapolated temperatures of the initiation and the termination of the above-mentioned glass transition is 20xc2x0 C. or less.
6. An oil extended rubber according to 5 above wherein,
a monomer forming the above-mentioned olefinically unsaturated nitrile monomer unit is at least one selected from the group consisting of acrylonitrile and methacrylonitrile;
a monomer forming the above-mentioned aromatic vinyl monomer unit is at least one selected from the group consisting of styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, xcex1-methylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 4-tert-butylstyrene and tert-butoxystyrene;
a monomer forming the above-mentioned conjugated diene monomer unit is at least one selected from the group consisting of 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and chloroprene.
7. An oil extended rubber according to 6 above wherein, the above-mentioned extending oil is at least one selected from the group consisting of an aromatic-based oil, a naphthene-based oil and a paraffin-based oil.
8. A rubber composition being characterized in comprising 100 parts by mass of a conjugated diene-based rubber which has a repeating unit comprising {circle around (1)} 1 to 30% by mass of an olefinically unsaturated nitrile monomer unit, {circle around (1)} 10 to 50% by mass of an aromatic vinyl monomer unit {circle around (3)} 19.9 to 88.9% by mass of a conjugated diene monomer unit and {circle around (4)} 0.1 to 10% by mass of a monomer unit having one polymerizable unsaturated group and at least one functional group selected from the group consisting of an amino group, a hydroxy group, an epoxy group, a carboxyl group and an alkoxysilyl group [where the total of {circle around (1)}, {circle around (2)}, {circle around (3)} and {circle around (4)} is 100% by mass], and whose Mooney viscosity [ML1+4 (100xc2x0 C.)] is 20 to 200, and 10 to 150 parts by mass of an inorganic filler.
9. A rubber composition according to 8 above wherein, the transition point of the above-mentioned conjugated diene-based rubber is xe2x88x9260 to 0xc2x0 C., and the difference between the extrapolated temperatures of the initiation and the termination of the above-mentioned glass transition is 20xc2x0 C. or less.
10. A rubber composition according to 9 above wherein,
a monomer forming the above-mentioned olefinically unsaturated nitrile monomer unit is at least one selected from the group consisting of acrylonitrile and methacrylonitrile;
a monomer forming the above-mentioned aromatic vinyl monomer unit is at least one selected from the group consisting of styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, xcex1-methylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 4-tert-butylstyrene and tert-butoxystyrene;
a monomer forming the above-mentioned conjugated diene monomer unit is at least one selected from the group consisting of 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and chloroprene.
11. A rubber composition according to 10 above wherein the above-mentioned inorganic filler is a silica.
12. A rubber composition according to 11 above further comprising a carbonblack.
13. A rubber composition according to 11 or 12 above further comprising a silane coupling agent.
14. A rubber composition according to 8 above further comprising 10 to 60 parts by mass of an extending oil based on 100 parts by mass of the above-mentioned conjugated diene-based rubber and the Mooney viscosity [ML1+4 (100xc2x0 C.)] of the above-mentioned conjugated diene-based rubber is 40 to 200.
15. A rubber composition according to 14 above wherein, the transition point of the above-mentioned conjugated diene-based rubber is xe2x88x9260 to 0xc2x0 C., and the difference between the extrapolated temperatures of the initiation and the termination of the above-mentioned glass transition is 20xc2x0 C. or less.
16. A rubber composition according to 15 above wherein,
a monomer forming the above-mentioned olefinically unsaturated nitrile monomer unit is at least one selected from the group consisting of acrylonitrile and methacrylonitrile;
a monomer forming the above-mentioned aromatic vinyl monomer unit is at least one selected from the group consisting of styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, xcex1-methylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 4-tert-butylstyrene and tert-butoxystyrene;
a monomer forming the above-mentioned conjugated diene monomer unit is at least one selected from the group consisting of 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and chloroprene.
17. A rubber composition according to 16 above wherein, the above-mentioned extending oil is at least one selected from the group consisting of an aromatic-based oil, a naphthene-based oil and a paraffin-based oil.
18. A rubber composition according to 17 above wherein the above-mentioned inorganic filler is a silica.
19. A rubber composition according to 18 above further comprising a carbonblack.
20. A rubber composition according to 18 or 19 above further comprising a silane coupling agent.
A rubber composition of the present invention has a satisfactory processability and gives a rubber product having a low rolling resistance, a vulcanized rubber having an excellent wet skid property, sufficient wearing resistance, tensile strength and the like, which is useful especially as a tire. A conjugated diene-based rubber and an oil extended ruber of the invention are useful in producing a rubber composition described above.
A conjugated diene-based rubber according to the invention is a copolymer obtained by copolymerizing an olefinically unsaturated nitrile monomer (hereinafter referred to as Monomer {circle around (1)}), an aromatic vinyl monomer (hereinafter referred to as Monomer {circle around (2)}) and a conjugated diene monomer (hereinafter referred to as Monomer {circle around (3)}), and preferably is a random copolymer.
Monomer {circle around (1)} may be used acrylonitrile, methacrylonitrile and the like. Among these, acrylonitrile is preferred. Any of these Monomers {circle around (1)} may be used alone or in combination of two or more.
The monomer unit formed by Monomer {circle around (1)}, in a repeating unit constituting the conjugated diene-based rubber, is in an amount of 1 to 30% by mass, preferably 3 to 30% by mass, more preferably 6 to 20% by mass. An amount of the monomer unit formed by Monomer {circle around (1)} less than 1% by mass leads to a poor dispersion of an inorganic filler, resulting in an insufficiently improved wearing resistance or the like of a vulcanized rubber obtained. On the other hand, an amount exceeding 30% by mass leads to a poor low temperature performance of a vulcanized rubber obtained.
Monomer {circle around (2)} may be used styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, xcex1-methylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 4-tert-butylstyrene, tert-butoxystyrene and the like. Among these, styrene is preferred. Any of these Monomers {circle around (2)} may be used alone or in combination of two or more.
The monomer unit formed by Monomer {circle around (2)}, in a repeating unit constituting a conjugated diene-based rubber, is in an amount of 10 to 50% by mass, preferably 15 to 40% by mass. An amount of the monomer unit formed by Monomer {circle around (2)} less than 10% by mass leads to a reduced wearing resistance of a vulcanized rubber obtained. On the other hand, an amount exceeding 50% by mass leads to a reduced repelling elasticity of a vulcanized rubber obtained, resulting in an increased tan xcex4.
Monomer {circle around (3)} may be used 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, chloroprene and the like. Among these, 1,3-butadiene and isoprene are especially preferred. Any of these Monomers {circle around (3)} may be used alone or in combination of two or more.
The monomer unit formed by Monomer {circle around (3)}, in a repeating unit constituting a conjugated diene-based rubber, is in an amount of 19.9 to 88.9% by mass, preferably 19.9 to 80% by mass. An amount of the monomer unit formed by Monomer {circle around (3)} less than 19.9% by mass leads to a reduced repelling elasticity of a vulcanized rubber obtained, resulting in an increased tan xcex4.
Among Monomer {circle around (4)}, a monomer having one polymerizable unsaturated group and an amino group is preferably a monomer having a tertiary amino group, including;
(a) dialkylaminoalkyl (meth)acrylates such as dimethylaminomethyl (meth)acrylate, diethylaminomethyl (meth)acrylate, 2-dimethylaminoethyl (meth)acrylate, 2-diethylaminoethyl (meth)acrylate, 2-dimethylaminoethyl (meth)acrylate, 2-diethylaminoethyl (meth)acrylate, 2-(d-n-propylamino)ethyl (meth)acrylate, 2-dimethylaminopropyl (meth)acrylate, 2-diethylaminopropyl (meth)acrylate, 2-(d-n-propylamino)propyl (meth)acrylate, 3-dimethylaminopropyl (meth)acrylate, 3-diethylaminopropyl (meth)acrylate and 3-(d-n-propylamino)propyl (meth)acrylate, and the like;
(b) N-dialkylaminoalkyl group-containing unsaturated amides such as N-dimethylaminomethyl (meth)acrylamide, N-diethylaminomethyl (meth)acrylamide, N-(2-dimethylaminoethyl) (meth)acrylamide, N-(2-diethylaminoethyl) (meth)acrylamide, N-(2-diethylaminopropyl) (meth)acrylamide, N-(3-dimethylaminopropyl) (meth)acrylamide, N-(3-diethylaminopropyl) (meth)acrylamide and the like;
(c) tertiary amino group-containing vinyl aromatic compounds, such as N,N-dimethyl-p-aminostyrene, N,N-diethyl-p-aminostyrene, dimethyl(p-vinylbenzyl)amine, diethyl(p-vinylbenzyl)amine, dimethyl(p-vinylphenethyl)amine, diethyl(p-vinylphenethyl)amine, dimethyl(p-vinylbenzyloxymethyl)amine, dimethyl[2-(p-vinylbenzyloxy)ethyl] amine, diethyl(p-vinylbenzyloxymethyl)amine, diethyl[2-(p-vinylbenzyloxy)ethyl]amine, dimethyl(p-vinylphenethyloxymethyl)amine, dimethyl[2-(p-vinylphenethyloxy)ethyl] amine, diethyl(p-vinylphenethyloxymethyl)amine, diethyl[2-(p-vinylphenethyloxy)ethyl] amine, 2-vinylpyridine, 3-vinylpyridine, 4-vinylpyridine and the like.
Among these listed above, a dialkylaminoalkyl (meth)acrylate and a tertiary amino group-containing vinyl aromatic compound are preferred. More typically, dimethylaminomethyl (meth)acrylate, diethylaminomethyl (meth)acrylate, 2-dimethylaminoethyl (meth)actylate, 2-diethylaminoethyl (meth)acrylate, 2-dimethylaminopropyl (meth)acrylate, 2-diethylaminopropyl (meth)acrylate, 3-dimethylaminopropyl (meth)acrylate, 3-diethylaminopropyl (meth)acrylate, N,N-dimethyl-p-aminostyrene, N,N-diethyl-p-aminostyrene, dimethyl (p-vinylbenzyl)amine and diethyl (p-vinylbenzyl) amine are preferred. Any of these amino group-containing monomers may be employed alone or in combination of two or more.
A monomer having one polymerizable unsaturated group and a hydroxyl group includes;
(a) hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate and the like;
(b) mono (meth)acrylates of a polyalkylene glycol (the number of alkylene glycol units is for example 2 to 23) such as polyethylene glycol, polypropylene glycol and the like;
(c) Hydroxyl group-containing unsaturated amides such as N-hydroxymethyl (meth)acrylamide, N-(2-hydroxyethyl) (meth)acrylamide, N,N-bis(2-hydroxyethyl) (meth)acrylamide and the like;
(d) hydroxyl group-containing vinyl aromatic compounds such as o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, o-hydroxy-xcex1-methylstyrene, m-hydroxy-xcex1-methylstyrene, p-hydroxy-xcex1-methylstyrene, p-vinylbenzyl alcohol and the like;
(e) (meth)allyl alcohols and the like.
Among those listed above, hydroxyalkyl (meth)acrylates and hydroxyl group-containing vinyl aromatic compounds are preferred. More typically, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, o-hydroxystyrene, m-hydroxystyrene and p-hydroxystyrene are preferred. Any of the hydroxyl group-containing monomers may be employed alone or in combination of two or more.
A monomer having one polymerizable unsaturated group and an epoxy group includes (meth)allylglycidylether, glycidyl (meth)acrylate and 3,4-oxycyclohexyl (meth)acrylate, and the like. More typically, glycidyl (meth)acrylate is preferred. Any of these epoxy group-containing monomers may be employed alone or in combination of two or more.
A monomer having one polymerizable unsaturated group and a carboxyl group includes;
(a) unsaturated carboxylic acids such as (meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, tetraconic acid, cinnamic acid and the like as well as a salt or the like thereof; and,
(b) free carboxyl group-containing esters such as a monoester or the like of a non-polymerizable polyvalent carboxylic acid such as phthalic acid, succinic acid, adipic acid and the like with a hydroxyl group-containing unsaturated compound such as (meth)allyl alcohol, 2-hydroxyethyl (meth)acrylate and the like as well as a salt or the like thereof.
Among those listed above, an unsaturated carboxylic acid is preferred. More typically, acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid are preferred. Any of these carboxy group-containing monomers may be used alone or in combination of two or more.
A monomer having one polymerizable unsaturated group and an alkoxysilyl group includes; (meth)acryloxymethyl trimethoxysilane, (meth)acryloxymethyl methyldimethoxysilane, (meth)acryloxymethyl dimethylmethoxysilane, (meth)acryloxymethyl triethoxysilane, (meth)acryloxymethyl methyldiethoxysilane, (meth)acryloxymethyl dimethylethoxysilane, (meth)acryloxymethyl tripropoxysilane, (meth)acryloxymethyl methyldipropoxysilane, (meth)acryloxymethyl dimethylpropoxysilane, xcex3-(meth)acryloxypropyl trimethoxysilane, xcex3-(meth)acryloxypropyl methyldimethoxysilane, xcex3-(meth)acryloxypropyl dimethylmethoxysilane, xcex3-(meth)acryloxypropyl triethoxysilane, xcex3-(meth)acryloxypropyl methyldiethoxysilane, xcex3-(meth)acryloxypropyl dimethylethoxysilane, xcex3-(meth)acryloxypropyl tripropoxysilane, xcex3-(meth)acryloxypropyl methyldipropoxysilane, xcex3-(meth)acryloxypropyl dimethylpropoxysilane, xcex3-(meth)acryloxypropyl methyldiphenoxysilane, xcex3-(meth)acryloxypropyl dimethylphenoxysilane, xcex3-(meth)acryloxypropyl methyldi benzyloxysilane, xcex3-(meth)acryloxypropyl dimethyliphenoxysilane, and the like.
Among those listed above, xcex3-(meth)acryloxypropyl trimethoxysilane, xcex3-(meth)acryloxypropyl methyldimethoxysilane xcex3-(meth)acryloxypropyl dimethylmethoxysilane, xcex3-(meth)acryloxypropyl triethoxysilane, xcex3-(meth)acryloxypropyl methyldiethoxysilane, xcex3-(meth)acryloxypropyl dimethylethoxysilane, xcex3-(meth)acryloxypropyl tripropoxysilane, xcex3-(meth)acryloxypropyl methyldipropoxysilane and xcex3-(meth)acryloxypropyl dimethylpropoxysilane are preferred. Any of these alkoxysilyl group-containing monomers may be used alone or in combination of two or more.
The monomer unit formed by Monomer {circle around (4)}, in a repeating unit constituting a conjugated diene-based rubber, is in an amount of 0.1 to 10% by mass, preferably 0.1 to 5% by mass. An amount of the monomer unit formed by Monomer {circle around (4)} less than 0.1% by mass leads to an insufficient affinity between the conjugated diene-based rubber and an inorganic filler such as a silica, resulting in an insufficient improvement in tan xcex4 and the wearing resistance. On the other hand, an amount exceeding 10% by mass leads to a firm aggregation between the conjugated diene-based rubber and an inorganic filler such as a silica, resulting in a poor processability.
A conjugated diene-based rubber may be obtained by copolymerizing various monomers if necessary in addition to Monomers {circle around (1)}, {circle around (2)}, {circle around (3)} and {circle around (4)}.
Such monomer may be used an ester-based monomer including a (meth)acrylate such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-amyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate and the like as well as a vinyl ester such as vinyl acetate or the like. While such ester-based monomer can be used in an amount by which the characteristics of the conjugated diene-based rubber is not affected adversely, the monomer unit formed by the ester-based monomer is preferably in an amount of 20% by mass or less based on the total monomer units.
The xe2x80x9cMooney viscosityxe2x80x9d [ML1+4 (100xc2x0 C.)] of the conjugated diene-based rubber is 20 to 200, preferably 30 to 150. The Mooney viscosity less than 20 leads to a reduced wearing resistance of a vulcanized rubber obtained. On the other hand, the viscosity exceeding 200 leads to a poor processability of a rubber composition containing this conjugated diene-based rubber.
While the xe2x80x9cglass transition pointxe2x80x9d of the conjugated diene-based rubber may vary depending on the composition of the monomers employed, it is preferably xe2x88x9260 to 0xc2x0 C., more preferably xe2x88x9250 to xe2x88x9210xc2x0 C., when determined by a differential scanning calorimeter (DSC) in accordance with ASTM D3418-82 (reapproved 1988). In addition, the difference between the extrapolated temperatures of the initiation and the termination of the glass transition is preferably 20xc2x0 C. or less, preferably 18xc2x0 C. or less. A difference exceeding 20xc2x0 C. leads a vulcanized rubber having a reduced wet skid resistance and an increased tan xcex4, which are not preferable.
The weight-average molecular weight of the conjugated diene-based rubber represented as polystyrene determined by a gel permeation chromatography (GPC) is preferably 100,000 or more, more preferably 100,000 to 2,000,000, particularly 200,000 to 1,500,000. The weight-average molecular weight less than 100,000 may lead to a reduced wearing resistance of a vulcanized rubber obtained, which may results in an increased tan xcex4. On the other hand, the molecular weight exceeding 2,000,000 may lead to a poor processability of a rubber composition containing this conjugated diene-based rubber. This weight-average molecular weight can be controlled during polymerization by using a chain transferring agent such as an alkylmercaptan employed commonly in a radical polymerization.
The conjugated diene-based rubber can be produced by polymerizing Monomers {circle around (1)}, {circle around (2)}, {circle around (3)} and {circle around (4)} optionally with an ester-based monomer in the presence of a radical initiator in an aqueous medium. While the polymerization method is not particularly limited, it is preferably an emulsion polymerization in an usual case. Such emulsion polymerization may be performed by any ordinary method, such as a method in which a certain monomer is emulsified in an aqueous medium in the presence of an emulsifier and the polymerization is initiated using a radical initiator and then terminated using an inhibitor once a predetermined conversion is achieved.
In a preferred manner of adding Monomer {circle around (1)}, its portion is added prior to the initiation of the polymerization and the remainder is then added to the polymerization system intermittently or continuously over the polymerization process, or its entire amount is added to the polymerization system intermittently or continuously over the polymerization process. It is not preferable to add the entire amount of Monomer {circle around (1)} prior to the initiation of the polymerization to effect a copolymerization, since the difference between the temperatures of the initiation and the termination of the glass transition of the copolymeric rubber tends to exceed 20xc2x0 C.
An emulsifier may be used an anionic surfactant, a nonionic surfactant, a cationic surfactant, an amphoteric surfactant and the like. A fluorine-based surfactant may also be employed. Any of these emulsifiers may be used alone or in combination of two or more. Among these emulsifiers, those employed frequently are anionic surfactants, such as a potassium or sodium salt or the like of a long chain fatty acid having 10 or more carbon atoms such as capric acid, lauric acid, myristic acid, palmitic acid, oleic acid, stearic acid, and the like, as well as a rosin acid salt or the like.
A radical initiator may be used an organic peroxide such as benzoyl peroxide, lauroyl peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, p-menthane hydroperoxide, di-tert-butyl peroxide, dicumyl peroxide, and the like. Those which may also be employed are an azo compound such as azobisisobutyronitrile, an inorganic peroxide such as potassium persulfate as well as any of these peroxides in combination with ferrous sulfate which is one of redox catalysts or the like. Any of these radical initiators may be used alone or in combination of two or more.
For the purpose of adjusting the molecular weight of a conjugated diene-based rubber, a chain transferring agent including an alkylmercaptan such as tert-dodecylmercaptan and n-dodecylmercaptan, carbon tetrachloride, a thioglycol, diterpene, terpinolene, xcex3-terpinene, and the like can also be employed.
A polymerization can be effected in an oxygen-free reaction vessel at a temperature of 0 to 100xc2x0 C., and an especially preferred polymerization temperature ranges from 0 to 80xc2x0 C. The polymerization may be of a continuous process or a batch process, and the operating conditions or the like such as the polymerization temperature or the like and the stirring manner or the like can appropriately be changed in the course of the reaction. Since a higher conversion of the polymerization may leads to a gelation, it is preferable to keep the conversion of the polymerization at 80% or less. The polymerization can be terminated, once a predetermined conversion of the polymerization is accomplished, by adding an inhibitor. Such inhibitor may for example be an amine compound such as hydroxylamine, diethylhydroxylamine, and the like or a quinone compound or the like such as hydroquinone or the like.
After terminating the polymerization, a resultant conjugated diene-based rubber latex is made free from unreacted monomers if necessary by means of steam distillation and the like, and then supplemented with a salt such as sodium chloride, potassium chloride and calcium chloride together with hydrochloric acid, nitric acid or sulfuric acid if necessary to allow the conjugated diene-based rubber to be aggregated as crumb. This crumb is washed, dehydrated and then dried with a drier or the like, whereby obtaining a conjugated diene-based rubber.
The oil extended rubber in the present invention contains a conjugated diene-based rubber and an extending oil.
The Mooney viscosity [ML1+4 (100xc2x0 C.)] of the conjugated diene-based rubber constituting the oil extended rubber is 40 to 200 and is preferably 70 to 170. The Mooney viscosity less than 40 leads to a reduced wearing resistance of a vulcanized rubber obtained. On the other hand, the viscosity exceeding 200 leads to a poor processability of a rubber composition containing this oil extended rubber.
The extending oil described above is not particularly limited, it may be used an aromatic-based oil, a naphthene-based oil and a paraffin-based oil. Any of these oils may be used alone or in combination of two or more. Among these oils, an aromatic-based extending oil is preferred particularly.
The extending oil described above may be present in an amount of 10 to 60 parts, preferably 20 to 50 parts, based on 100 parts by mass (hereinafter abbreviated as parts) of the conjugated diene-based rubber. An amount of the extending oil less than 10 parts leads to an insufficient improvement in the processability, while an amount exceeding 60 parts is also problematic since the ratio of the extending oil to be incorporated during the preparation of the rubber composition for obtaining a required processability is limited. The Mooney viscosity [ML1+4 (100xc2x0 C.)] of an oil extended rubber obtained is preferably 20 to 180, more preferably 30 to 150.
The oil extended rubber in the invention can be obtained by mixing an aqueous solution of an emulsifier and an extending oil, preparing an emulsion of the extending oil for example by stirring, mixing with a conjugated diene-based rubber latex, and then aggregating by the method described above.
The rubber composition in the present invention contains a conjugated diene-based rubber and an inorganic filler. The Mooney viscosity [ML1+4 (100xc2x0 C.)] of the conjugated diene-based rubber constituting the rubber composition is 20 to 200 and is preferably 30 to 150. The rubber composition can be that further includes an extending oil. The extending oil described above may be present in an amount of 10 to 60 parts, preferably 20 to 50 parts, based on 100 parts by mass of the conjugated diene-based rubber. When the extending oil is employed, the Mooney viscosity [ML1+4 (100xc2x0 C.)] of the conjugated diene-based rubber is preferably 40 to 200.
Other conjugated diene-based rubber may additionally be employed as a rubber component constituting the rubber composition. Such rubber component may be used a styrene-butadiene copolymeric rubber, a butadiene rubber, an isoprene rubber, a butadiene-isoprene copolymeric rubber, a butadiene-styrene-isoprene copolymeric rubber, an acrylonitrile-butadiene copolymeric rubber, a natural rubber, a chloroprene rubber and the like.
An inorganic filler may be used silica, aluminum hydroxide, composite oxide contained silicon oxide such as a composite oxide formed by silicon oxide and at least one selected from aluminium oxide, calcium oxide and magnesium oxide, zinc oxide, clay, calcium carbonate, magnesium carbonate and the like. Among these, silica is prefered. Such silica is not particularly limited and may be any one used generally as a bright color reinforcing agent for a synthetic rubber. While the type of a silica is not particularly limited, a wet process type white carbon, a dry process type white carbon, a colloidal silica and a precipitated silica and the like described in Japanese Patent laid-open publication 62-62838 can be employed. Among those listed above, a wet process type white carbon whose main component is silicic hydrate is preferred particularly. Any of these silica-based compound may be employed alone or in combination of two or more. While the specific surface area of the silica is not particularly limited, a nitrogen absorption specific surface area (area determined by BET method in accordance with ASTM D3037-81) is usually 50 to 400 m2/g, preferably 50 to 220 m2/g, more preferably 70 to 220 m2/g, for the purpose of achieving a sufficient improvement in the reinforcing performance, the wearing resistance and the exothermic behavior, and the like.
The inorganic filler such as a silica or the like may preferably be present in an amount of 10 to 150 parts, particularly 20 to 100 parts based on 100 parts of a total rubber component. An amount of the inorganic filler less than 10 parts leads to an insufficient reinforcing effect, resulting in a poor wet skid property of a vulcanized rubber obtained or the like. A sufficient reinforcing effect can be accomplished when the filler amounts to 150 parts, and no further amount is required.
A carbon black, carbon-silica dual-phase filler and the like may be also added for purpose of reinforcing the rubber product.
While the type of the carbon black is not particularly limited, those which may be employed are a furnace black, an acetylene black, a thermal black, a channel black, a graphite, and the like. Among those listed above, a furnace black is preferred including SAF, ISAF, ISAF-HS, ISAF-LS, IISAF-HS, HAF, HAF-HS, HAF-LS, FEF, and the like. Any of these carbon blacks may be employed alone or in combination of two or more.
While the nitrogen absorption specific surface area of a carbon black determined similarly to a silica is not particularly limited, it is usually 5 to 200 m2/g, preferably 50 to 150 m2/g, more preferably 80 to 130 m2/g, for the purpose of achieving a sufficient improvement in the tensile strength and the wearing resistance of a vulcanized rubber and the like. While the DBP absorption level of the carbon black is not particularly limited as well, it is 5 to 300 ml/100 g, preferably 50 to 200 ml/100 g, more preferably 80 to 160 ml/100 g, for the purpose of a sufficient improvement in the tensile strength and the wearing resistance, and the like. A further improvement in the wearing resistance can be achieved by using as a carbon black a high structure carbon black described in Japanese Patent laid-open publication 5-230290, whose cetyltrimethylammonium bromide absorption specific surface area is 110 to 170 m2/g and whose DBP (24M4DBP) oil absorption level after compressing under 165 MPa four times repetitively is 110 to 130 ml/100 g.
A carbon black may be employed with the inorganic filler described above. When the inorganic filler is a silica, the total amount is preferably 10 to 150 parts, especially 50 to 100 parts based on 100 parts of the conjugated diene-based rubber in the case of employing the silica and the carbon black concomitantly. A total amount less than 10 parts leads to an insufficient reinforcing effect, resulting in a poor wet skid property of a vulcanized rubber obtained or the like. A sufficient reinforcing effect can be accomplished when the total amount is 150 parts, and no further amount is required. While the ratio between a silica and a carbon black is not particularly limited, 100 parts of the silica is combined preferably with 5 to 30 parts, especially 10 to 15 parts of the carbon black. Within this range of the ratio, a vulcanized rubber which is excellent in all of the wet skid property, the repelling elasticity and the tensile strength can be obtained.
In addition to the components described above, other additives such as vulcanizing agents, silane coupling agents, anti-aging agents, processing aids and the like may also be employed.
While a representative vulcanizing agent is sulfur, other materials such as sulfur-containing compounds and peroxides or the like may also be employed. Such vulcanizing agent is present preferably in an amount usually of 0.5 to 10 parts, especially 1 to 6 parts, based on 100 parts of a rubber component.
When using a vulcanizing agent, a vulcanizing accelerator, a vulcanizing aid and the like may also be employed. Such vulcanizing accelerator may be used an aldehyde ammonia-based vulcanizing accelerator such as hexamethylenetetramine;
a sulfenamide-based vulcanization accelerator such as N-cyclohexyl-2-benzothiazolesulfenamide, N-tert-butyl-2-benzothiazolesulfenamide, N-oxyethylene-2-benzothiazolesulfenamide and N,Nxe2x80x2-diisopropyl-2-benzothiazolesulfenamide, a guanidine-based vulcanization accelerator such as diphenylguanidine, diorthotolylguanidine and orthotolylbiguanidine and a thiourea-based vulcanization accelerator such as thiocarboanilide, diorthotolylthiourea, ethylene thiourea, diethylthiourea and trimethylthiourea;
a thiazol-based vulcanization accelerator such as 2-mercaptobenzothiazol, dibenzothiazyldisulfide, 2-mercaptobenzothiazole zinc salt, 2-mercaptobenzothiazole sodium salt, 2-mercaptobenzothiazole cyclohexylamine salt and 2-(2,4-dinitrophenylthio)benzothiazole and thiuram-based vulcanization accelerator such as tetramethylthiuram monosulfide, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide and dipentamethylenethiuram tetrasulfide;
a dithiocarbamate-based vulcanization accelerator such as sodium dimethyldithiocarbamate, sodium diethyldithiocarbamate, sodium di-n-butyldithiocarbamate, lead dimethyldithiocarbamate, zinc dimethyldithiocarbamate, zinc diethyldithiocarbamate, zinc di-n-butyldithiocarbamate, zinc pentamethylenedithiocarbamate, zinc ethylphenyldithiocarbamate, tellurium diethyldithiocarbamate, selenium dimethyldithiocarbamate, selenium diethyldithiocarbamate, copper dimethyldithiocarbamate, iron dimethyldithiocarbamate, diethylamine diethyldithiocarbamate, piperidine pentamethylene dithiocarbamate and pipecolin methylpentamethylene dithiocarbamate and xanthate-based vulcanization accelerator such as sodium isopropylxanthate, zinc isopropylxanthate and zinc butylxanthate, and the like.
Any of these may be used alone or in combination of two or more. This component may be present preferably in an amount of 0.5 to 15 parts, especially 1 to 10 parts based on 100 parts of a rubber component.
When a silica is added to a rubber composition of the invention, the wearing resistance and the tan xcex4 of a resultant vulcanizing rubber can further be improved by further adding a silane coupling agent.
The silane coupling agent is not particularly limited, it may be used vinyltrichlorosilane, vinyltriethoxysilane, vinyltris(xcex2-methoxy-ethoxy)silane, xcex2-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane, xcex3-glycidoxypropyl trimethoxysilane, xcex3-glycidoxypropylmethyl diethoxysilane, xcex3-methacryloxypropyl trimethoxysilane, N-(xcex2-aminoethyl)-xcex3-aminopropyl trimethoxysilane, N-(xcex2-aminoethyl)-xcex3-aminopropyltrimethyl dimethoxysilane, N-phenyl-xcex3-aminopropyl trimethoxysilane, xcex3-chloropropyl trimethoxysilane, xcex3-mercaptopropyl trimethoxysilane and xcex3-aminopropyl trimethoxysilane.
Those which can also be employed are bis-[3-(triethoxysilyl)propyl] tetrasulfide, bis-[3-(triethoxysilyl)propyl] disulfide, xcex3-trimethoxysilylpropyl dimethylthiocarbamyl tetrasulfide, xcex3-trimethoxysilylpropyl benzothiazyl tetrasulfide, and the like. Any of such silane coupling agents may be used alone or in combination of two or more.
A silane coupling agent is present preferably in an amount of 1 to 20 parts, especially 2 to 15 parts, based on 100 parts of the silica.
A rubber composition according to the invention and a rubber product employing the same can be produced as described below.
First, a rubber component comprising a conjugated diene-based rubber or an oil extended rubber, a silica, a carbon black, a reinforcing agent such as a carbon-silica dual phase filler or the like, a rubber extending oil and other auxiliary components or the like are kneaded using a kneader such as Banbury mixer or the like at a temperature of 70 to 180xc2x0 C. Subsequently, the kneaded mixture is cooled and combined with a vulcanizing agent such as a sulfur or the like and a vulcanization accelerator or the like using Banbury mixer, a mixing roll, and the like to mold into a desired shape. A vulcanization at a temperature of 140 to 180xc2x0 C. is then effected to obtain a desired vulcanized rubber, i.e., a rubber product.
A vulcanized rubber obtained by employing a rubber composition according to the invention has excellent wet skid property, repelling elasticity, and the like together with sufficient wearing resistance, tensile strength, and the like as well as a satisfactory processability, thus being useful as a rubber composition for a tire, especially as a tire tread.
A vulcanized rubber obtained by employing a rubber composition according to the invention has excellent wet skid property, repelling elasticity, and the like together with sufficient wearing resistance, tensile strength, and the like as well as a satisfactory processability, thus being useful as a rubber composition for a tire, especially as a tire tread.